Assigwob to westinghottse



H. A. LEWIS. ELECTROMAGNET APPARATUS. APPLICATION FILED MAY 22. m1.

4 SHEETSSHEEI l- 34 3 z M a 17 29 WITNESSES: I

m ENTOR 7 AITTURNEY Patented May 11, 1920.

H. A. LEWIS.

ELECTHOMAGNET APPARATUS.

APPLICATION HLED MAY 22,1911.

1,339,900, Patented May 11, 1920.

' 4 SHEETS-SHEET 2. 6 J3 J2 0 0 Km i WITNESSES: INVENTOR 7 ATTORNEY H.A. LEWIS.

ELECTROMAGNET APPARATUS.

APPLICATION FILED MAY 22. 19M.

1,339,900. Pawnm y 1920.

I45 4 snms s ,46

WITNESSES: INVENTOR 'ATTORNEY H. A. LEWIS. ELECTHOMAGNET APPARATUS.APPLICATION FILED MAY 22. 19M.

Patented May 11, 1920.

4 SHEETS-SHEET 4.

A'TTOHLEY VENIOR WITNESSES:

- above referred to.

names .A.- Lewis, or nonms'rowmirsmn SYLVANIA, ASSIGNOR T0 "WESTINGHOUSEELEGTRIC' ANDTMANUEACTURING COMPANY. A CORPORATION OF PENNSYLVANIA.

'ELECTROMAGNET APPARATUS.

1 ,s-saaoo.

.-Speoifieat1on of Letters Patent.

Patcnted May 11,?1920.

H Application flld'May 22, 1911. Serial No. 628,733.

To 'all whom't'tmay'concew-m 'Be'it known that I Hahn A. 'Lnw1s,"acitizen of the United tates, and a' resident or Norristown, in'the countofMontgom cry and State of Pennsylvan a, have invented anew and.usefullmprovement infElectromagnet Apparatus; of which the followin isa specification.

y inve n "on relates to. electro magnets and eleetromagneti'ca'lly.actuated switches for the control of electric circuits. I

One .object of. my invention is to, provide anelectro-magiiet"comprisinga single coil or a plurality of cooperatingcoils and a movable armaturelorcore'that is actuated onl by theenergization ofsaid coil or colls wit liIl predetermindlimits. A secondobject of .Iny invention isto provide an electricswitching jdevicethatis actuated an electro-magnet having the characteristics Torconvenience, a switch of this character .will hereinafterhe re ferred'to as a'magnet switch. 1

Control systems dffthe unit-switch type have been l rgelyusedgforfgoverning electric motors andhav e comprised a plurality ofindependently operated switches designed and connected toactsuccessiyely to produce a, gradualand. automatic acceleration of themotors with which they have been associated. In these systems,someindependent means is ordinarily employe'dforf delaying the action ofthesuccessive switches-in order'to ayoid injuring the" motor by excessivecurrent. I

;Vhile my present inventio-mbroa'dly con sidcred, consists of anelectromagnetic device having peculiar -of crating characteristics, in amore limite .aspect,,it consists of a .magnet switch that isparticularly adapted for useiin unit-switch control systems.

The magnet switch of my invention has the characteristic of occupyingits'o ')en position, not only when its; actuating magnet is'deenergized. but/also when it is energized above a predetermined value.In other words, the. switch is closed (my when energized betweenv,p'ra'dete miner. limits. By utilizing @a Iplurality of switches ofthis character in a {simple unit-switch control.

system, the facceieration of an electric fmotor may be {advantageously"accomplished and'the action of each switeh 'delayed until the currenttraversing the motor circuit has su'fiiciently decreased to fall withinthelim its which determine the action of the switches.

in addition to the above-named characteristlcs, the switch, when closed,will remain closed until its actuating magnet winding is denergized,unless the current traversing the coil becomes excessive.

'Figure 1 of the accompanying drawings is a partially sectionalelevationof a magnet switch havingthe operating character'- istics of myinvention and arranged and constructed in accordance therewith. "Fig. 218 a similar view of a. modified'structure also embodying my invention.Figs. 3 mm are diagrams illustrating the application of the switchesshown respectively in"Figs..l and 2 to. simple systems of electric motorcontrol. Fig. 5 is anfelevation and Fi'g. 6 is a plan View, partially insection, of a magnet switch that is similar tothe switch of igh2, exceptthat four, in'steaid of two, independently. movable core members .orarmatnres are provided.

Referring to Fig. 1 of the drawings, the switch here shown comprises astationary magnetizable member 1, movable magnetizable members 2 and 3,stationary switch contact members 4 and 5, movable switch contactmembers 6 and T and actuating coils 8 and 9.

The stationary member 1 is rectangular in form andcomprises a centralbranch IO'of relativelylarge cross section .on which the coils 8 and 9.are'both mounted, and side branches 1] and 12, whichare connected,through the branches 28 and'QQ", to the ends of the branch '10 outside,of the coils '8' and 9 and are inset at 13 and 14Jb'etwee'n the coilsso that they come into contact,,or si1bstantially into contact, with thecentral branch '10, at these points. The portions of the side branches.11 and 1-2 which'e'xtend to the inset portions 13 and'14 are, forconvenience, labeled 29 and 28".

The movable members '2 and 3 are alike and eomprisearms15- and 1'6thatfarejpivotally secured tothefstationaryfmember 1 at 17 and1-8 andare disposed substantially parallel to, the side branchesll-a nd 12."The said arms are severally provided with lateral projections 19 and QOthat extendin o recesses 21 an'd22 formed by the insetpor conjunctionterial, it will tions 13 and 14 of the side branches 11 and 12 of thestationary member 1; inwardly projecting arms 23 and 24 andcontact-bearing arms 25 and 26', to which the members 6 and 7 areattached. The widths of the air ga s between the ends of the projections19 an 20 and the inset portions 13 and 14 are determined b adjustingscrews 27 and 2.7 with which the lower ends of the arms 15 and 16 areprovided.

The coils 8 and 9 are connected to act in with each other electrically.as a single coilflo yt'hey may have different structural and functionalcharacteristics; for example, the coils may be parts of a single serieswinding, or the coil 8 may be a hi h resistance shunt coil supplied withsmal current, while the coil'9 may be a low-resistance series coil. I

The magnetic fluxestablished by the coil 8 traverses the circuitscomprising the central branch 10,,the side branches 11 and 12 and thebranches 2S'Q a'nd thegportions 29. That'portionof the stationarymagnetizable member lwliieb isincluded in the circuits becomes energizedto attract the armatures 23 and 24,-{a'ndbeing.ofmaghetizable matend tomaintain the fluxwith: in its own confines. If the flux,,however, isabove apredetermined value, sufliciently large to saturatetheabove-traced circuits, it will traverse additional circuitscomprisingthe central hranch'lO, the, insetportions 13 and 1 1, therecesses 21 ,and22, the side branches 1]. and and th'e branches 28". Thefiux-will leak be ond}thc,confines of the circuits justrtrace to 'enterthe projections 19. and 20, the armslaand 16 and the armatures 2 3-and,24.' leakage .flux will aid th'elrna'grietism of the saturated circuitsto close the 'switchfmembers. The flux will even traverse. the full lenh ofthe central branch 10,' continuing y wa of the branches 29 .and thelower portions of the branches ll (1 12, across the recesses 21 and22.],This am is rendered particularly easy of travel becausethe flux ofthe coil 9 -is in the samei'direction as, that of the coil 8. Theexcessor. leakage fluxtraversing the recesses2'1 and 22 will ,of course,act upon the pro'ections 19, and 20 with forces op-. posing' t eforc'esacting upon the armaturcs Themagnetic flint. set into 0 oration by theshunt coil ,8- is thusiadapte to actuate the switch members 7 and, 6,throu h its in? fluence upon thearmatures 23 an 24, and to-oppose ,.suchactuatiomithro h. its influence upon the projections 19 an 20. This isa. veryimportant feature; for variations in line voltage or in-the temerature of the shunt coil8 result in varying 0th the forces a'ctinguponthe armatultes 23 and 24 and the projections 19 and 20, so. that thetime of closing of the switch members 7 and 6 is almost whollindependent of such variations, being ependent upon the decrease in theflux of the series coil 9 alone.

The operation of the electro-magnet and of the switch contact membersactuated by it is as follows:

Assuming that the coils 8 and 9 are independently connected and only thecoil 8 is energized, the arrangement of parts is such that the arms 23and 24 will be attracted to the adjacent end of the branch 10 and theswitch contacts 6 and 7 will be moved into en agement with thestationary members 4 an 5. If, however, the coil 9 is strongly energizedwhile energy is supplied to the coil 8, a leakage ma netic flux willtraverse the air ga s provi ed by the screws 27 and 27 and sit er thearms 15 and 16 and the central branch 10 or portions of the arms 15 and16 and the projections 19 and 20 that will be of suflicient strength tooverbalance the pull upon the arms 23 and 24 and prevent the closing ofthe switch contact terminals until the current in the coil 9 falls belowa predetermined value. lVhen the said current reaches such predeterminedvalue, one of the movable members 2 and 3 will be actuated by themagnetic pull exerted upon the arms 23 and 24 and the correspondingswitch will be closed. It is evident that, if only a single pair ofswitch contacts is desired, only one of the movable members 2 and 3 isnecessary. Theoreticall both of the movable members can be ma e to actat the same time'by adjusting the sin ga s with sufficient care, but,practically, it IS impossibleto maintain such an action underordinary-operating conditions and, consequently, I prefer to use asingle movable member for one set of switch contacts and, if two or moremembers are emplo ed, to so adjust the air gaps that they wil actsuccessivcl to close two independent switches instead 0 acting together.

In the modificdstructure shown in Fig 2, which operates onthe sameprinciple asthe structure shown in Fig. 1, only a single coil 25 isemployed. The switch comprises, in

. addition to the coil 25, a central core member 26 on which the coil ismounted, a U-shaped member 27, transverse members 28 and 29 located atthe respective ends of the member 27 and movable core members 30 and 31.

Movable switch contact members 32 and 33 are respectively attached .tothe upper ends of the members 30 and 31 to cooperate with stationarycontact members 34 and 35. The movable core members or armatures 30 and31 are-pivotally attached to the respective ends of the member 29 at 36and 37, the switch contact members bein separated, when the coil 25 isdeenergize by springs 38 and 39 which hold the armatures 30 and 31 closeto the adjacent ends of the member '28. The armatul-es are preventedfrom comi'flg" i'nto-engagement= with the memher eab means 'f screws -a!id, 41'*tha6 ina'y be edjusted to vary theaiisgap's between the 'sa-idparts. l he ercsssectional area15fthe transterse merhbe1-"29is"materially? less than the edms' miamg'areaefi thekU shaped member 27an the-*ei rtreii'fitiesof tlie-niembers arid-'4;l ar relauwi closeafidf partner to the-member 29*s'o-tliat, when theieeil esjis energized,thre -is'always a fiux'tending to osdfl1ate= the aiinatiires 'abonttheir pii'otal points-arid to%clos'etlie Switches;

mein'ber29 and the' lower ends ,ofthe armatures 30 and 31. Thisflux'obiiouslytends tol'hold the "Switch open since he air. gaps and f 43"arematerially farther fromthe piv otaljpoints 36faiidi37 thanfthe point ofapplication-6i there-sees which tend to close the switch. I Iftheeoil;.25 is connected in seriesflwith" imbwfiqpfl eimitggfid theswitch'eonmcflteiminms areaijra'nedto ex elude motor-startingresistance, it" "evidently possible toflso desrgnf the poi 1 25 and itscore members that" theifit'st rush of current, when the motor circuit;switch is closed, will prevent the immediate? closing of the swathcontacts and, as theniotorlaccelefites and the 'current 'dies'down,thearmature 30,

havingf'the larger air gaps lat 42, will be actuated and the contactterminals 32 and 34 be" brought into engagement with-- each other. It isalsoevident that another rush of current will thenresufltwhi'ch willtend to still further 'delaythe closing of the switch contactterminals33and 35. I The operation ofjthe switches'shown .in Figs. 1 arid2 will beconsidered more in detail in, connection with thediagrams .,of Fi .3 and'4 towh'ich reference will now be ad.

In Fig. 3 energy'is supplied to'motor 44 "from any suitable source of"electricit through line conductors '45' and46, the 'direction.0fmotor-rotation being controlled by reversing switches 47,48, 49'and"50,and the acceleration of'the motor being controlled 'byflresistor'sections" 51, 52:53 and 54 and magnet switches 55 and 56 which severallycorresporidto theswitch of'Fjg. 1. Assuming that the switches occupy thepositions shown in the drawing, if master switchi'57 is moved intoposition 1), energy is supplied from conductor 46, through conductor'58,switch 57 conductor 59, coils-60 and 61 of switches 4 and 50, conductor62, switch 57 and conductor 63 to the opposite line conductor 45. Thereversing switches 47 and'50 are immediately closed-arid the motorcircuit is estblishedfrom line conconductor 71, resistor sections 51 to54, in-

magnet winding 75,- condubtor"76,woverload release circuit breaker -77,"one blade of switch 64an'd conductor 78 to opposite line c'on'du'ctor45. 'Themagnet'-switches 55 and '56 are "respectivelyprovided with shuntcoils 79 'and"80 which are supplied with energy from conductor 46,through conductor'65,' relay switch 81 (which is closed when switch 49is open) and relayfs'witch 70, circuitlibeing completed fromvoppo'siteiterininalz'of e'o'il79' to the middle point of theaccelerating resistance and from the opposite terminal of coil .80 toone terminal of the resistance.

As'already explained in. connection with Fj gl, the"energy'supplied tothe coi1s'79 in-3'7 tends m anness arinatures 6f the switches'55an'd56,j but the jactio'n ofthe series coils 72"and 73,"althongh"theyare connected electrically in the-same direction as coils'gSOand79,tnds" to' J'revent movementof the armatures until he .motor currentfalls belowa predeterminedvalue. The action of the magnet switches maybe prop erly "adjusted. by Varying the number of turns in'the windings72, 73, 79 and"-but I prefer to make the switches identical structurallyand to produce the desiredaction, as hereinafter pointed out. Byconnecting the shunt coils to difi'erent points in the a'ccelcratingresistance, a higher volta e is agpliecl'to the one'than tothe' ot er.shown .in the drawings, the highervoltzige is applied to the coil 80and, consequently, the magnet switch 56 has a greater tendency to moveits armaturesthan has the switch 55. The airkgaps existing at 82 and 83are soa'djusted'that, as the motor speedincreases and the currenttratersing the-motor circuit is decreased, the armature "84 of switch 56is first 'actuatedand switch 85 is closed. As soon as this switch isclosed; resister section 54 is short circniteiand'the motor current isagain increased-so that the armature 86 is notmctuated untilthe motor isstill further accelerated. When the armature86 finally movcs,"switch 87is closed and both resistor sections 53 and 54 areshort circuited. Thevoltage applied to the coil 79 of magnet-switch 55 has conse uently beenincreased and, upon the next re notion of motor current armature 88 willbe actuated and resistor sectionu52 be short circuited, armature 89 andresistor section 5], being finally short circuited by-reason ofanadjustment of the air gaps of the switch 55 corresponding to that of theair gaps '82 and 83of switch 56.

y The relay switch forms no part of my present invention and is soarranged as to re-insert the accelerating resistance in case of apredetermined overload current traversing the motor circuit. If themotor current becomes very excessive, the overload release circuitbreaker 77 will be actuated to ositively;interrupt the circuit.Referring to F ig. 4 of the drawings, the system .hereshown correspondsto the system of Fig. 3 exce t that a reversin switch 90 is substituted:or the switches 4 to 50, inclusive; and magnet switches 91 and-92,which-correspond to the switch of Fi 2, are substitutedfor the switches55 an 56. Corresponding parts in the two systems are designated by the.same reference characters. The operation of the system of Fig. 4 is asfollows: Assuming that master switch 57 is moved topesition (1, energy,is supplied from line. conductor 45, through the master switch 57,vconductor 93, coil 94 of the re- ,versin'g 0, conductor 95 ,sw1tch 57 anc nductpr, 6 in). 9.11995 e sa e ,tor 46. Wheii coil94 is'energi'zedreversing switch arins' gfja nd 98a'r'e moved in opposite directionsaway from #8 11", other, andthe t r c rcus; mple e f o conductor45fthrou' hIc nductorfQQ the field-magnet winding 5, conductor 100,'switch contact 97, conductor 101, motor armature 67, conductor 102,switch contact 98", conductor 103, series coils 104 and 105, resistorsections 51 to 54,-'inclusive, and conductor 106 to the other-lineconductor 46. Since the currenttraversesthe coils 104 and 10 5, themagnet switches 91 and 92 will-be quite strongly energized when ,thereversing switch closed and the motor circuit isfirst established, then,asthe motor speed increases,' the mtor' current will be materiallyreduced until the armature 107 of the switch 92 is actuated nd theswitch 108 is closed in accordance with the principles of operationoutlined in connect1on .w1tl 1 the description of the device shown 1nFig. 2. As' sqon'as switch 108 is closed, resistor section 54 is shortcircuited and the resultin increasein the motor; current serves to stifurther delay the action of the armature 109 of the switch 92 and thearmatures 110 and 111 of magnet switch '91, which will operatesuccessively in the order named to short circuit the resistance sections53, 52 and 51. The armatures are made to operate in the propersuccession by adjustin the air gaps as explained in connectien with themagnet illustrated in Fig. 2, but it 'ma also be found desirable toprovide the coi s 104 and 105 with different numbers of turns.

Referring to Figs. 5 and 6 of the drawings the m net switch here shownis similar to that s own in Fig. 2 of the drawings parts are indicatedby and corresponding the same reference characters except that there arefour branches 27 and four independently movable core members orarmatures 30, the end member 28 being cruciform, as shown in Fig. 6.

The air a s existin between the memhers 30 an the ends 0? themember 28are adjusted so that the four armatures operate in a predeterminedsequence. Movable switch contact members are secured to the armatures 30and are adapted to cooperate with stationary contact members as in Fig.2. By means of the arrangement shown m Fig. 5', four independentlyoperated accelerating switches of theiusual type ma be replaced by asingle magnet switch o the type here shown and, since four acceleratingste -s are suflicient in a great many instances, it is evident fthataver simple controller embodying only asingl eEmagnetic switch maybeemployed. a Y Y It is evldent that magnet switches embodying. myinvention may be adapted'for use with aiv'ariety ot's'ys'tems oficontroland that 'th'estructure otthe switches themselves ma'v be 'i iaterial'lyvaried ',within the spirit ands'cop'e o't my-invention, since I believethat I an'i the first and original inventor of magnetic "ces having theoperating characteristics outlined above. Y I claim asmy'finvention:

1. An -electromagnet comprising a stationary magnetizable member, amovable magnetizable member,and two coils forvenergizing said, memberswith magnetism of the samepolarit said ma netism having a norrnal fluxpat that tends r01 roduce relative movement between said mex'n ers and'a leakgc finx path that tends to prevent relative movement between saidmembers.

2. The combination with a movable element, of means for actuating saidelement and a magnet the leakage flux of which is adapted to prevent theactuation'of said element. f

, 3. The combination witlra movable member, of a plurality of means forcreatin a ma etic. field for controllin said member an meanscoritrolled'bythe eakage flux of said field for, preventing theactuation of said m'embera v 4. The combination with a movable member,of means for'creatin a plurality of ma netic fields for control mg saidmember an means controlled by the leakage flux of said fieldsforpreventing. the actuation of said member. y

5. A magnet comprising two coils and a magnetizable member havm twopositions, the leakage flux-due to one 0 said coils tending to maintainsaid member in one of said positions and the flux due to the other coilserving to maintain said member in said other position.

6. An electromagnetic switch having a series coil and a shunt coil,means whereby theleakage flux due to said series coil tends to maintainsaid switch openl; and means whereby the iflux due to said shunt coilserves to maintain said switch closed.

7.. A controlling device for electric circuits comprisinga switchmembernormally biased; toy open position, a plurality of meansforacreating magnetic field, and means whereby the-lcakage flux of saidfield holds the switch member in open position.

8. Arr electromagnetic switch; having two coils for creating, twosupplementing. mag netic fields, one of said fields opposing the closingof said switch and the otherof said fields servingto hold said switch inclosed position.

9. The combination with a movable memr ber, of two coils forcreatingntwo supple. menting. magnetic circuits for controlling saidmember, the leakage flux of one of said circuits preventing theactuation of said member and said other circuit serving .to hold saidmember in actuated position.

10. An. electric switch having two coils, means whereby the leakagefiuxsproduced by onecofsaid coils tends to maintain said switch openandmeans whereby the leakage flux produced by the other of said coils tendsto closesaid switch.

11. The combination with a movableielement, of a magneti forexertingopposing forces upon said elementi-ande magnet the leakage flux of whichis adapted to additionally act upon said element in the direction of oneof said forces.

12. The combination with an electric motor having an armature, and asource of current for said motor, of a switch for controlling saidmotor, a magnetizable member for controlling said switch. a coilconnected to said source for controlling both ends of said member, and acoil in series with said armature for controlling one end only of saidmember.

13. The combination with a movable member having two magnetizableportions, of a shunt coil for both of said portions and a series coilfor one only of said portions.

14. An electromagnetic switch provided with a coil having shuntcharacteristics and a coil having series characteristics for creatingtwo magnetic fields adapted to act cumulatively to maintain said switchopen in opposition to the force of the field of said coil having shuntcharacteristics.

15. An electromagnetic switch comprising an armature, a lockout coilacting upon said armature to maintain said switch open and a closingcoil acting oppositely upon said armature with forces adapted,respectively, to aid said lockout coil to maintain said switch open andto close said switch.

16. The combination with a magnetizable frame having a recess, of anarmature having a projection adapted to extend into said recess andmeans for energizing. said frame to act oppositely upon: said armatureand said proj ection. E

17. The-combination withi a pair of relatively movable magnetizablemembers, of a series coil anda shunt COIlfOL energizing. said.magnetizable members withmagnetismof one polarity,

normal flux-path that tends to produce rela tive movement between saidmembers and a leakage .fiux path. that tends to. prevent rela tivemovement between said members.

18, Thecombination with .a pairof co-- axial, separately mounted coils,of an armature of U-shape one of the legs of .which ex tends betweensaid coilsand the other legof which is adapted to be attracted by one ofsaid coils.

19.1nasystem of motor control, the combination with a motor resistance,of a magnet the resistance having a stationaryimagnetizabie 1 member,

contact members i secured thereto and a voltagezcoil 1 tending. toactuate the movable members and close the switches, and-acurrentcoilioperating in thesame direction electrically but opposing the actionof the movable. members exceeds a predetermined amount.

20. In a system of motor control, the. cont bination with a motor havingan armature and an accelerating resistor in circuit therewith, and asource of current for said motor, of a magnet switch for governing saidresistor having a coil in series with said armature, means whereby theleakage flux of said series coil maintains said switch open when thecurrent traversing said motor exceeds a predetermined value, and a coilconnected to said source for maintaining said switch closed.

21. The combination with a movable element, of means tending to maintainsaid element in a predetermined position and a magnet the leakage fluxof which is adapted to prevent the actuation of said element to saidpredetermined position.

22. The combination with a movable element, of an electromagnet forexerting opposing forces upon said element and an electromagnet foradditionally acting upon said element in the direction of one of saidforces.

23. The combination with a movable switch member, of a series coil and ashunt coil for controlling said member, each adapted to create amagnetic flux, the magnetitflux of said shunt coil having a normal fluxpath distinct from the flux path of said series coil. and a leakage fluxpath that coincides in part with said flux path of said series coil, theflux traversing said normal flux path bcin adapted to effect the closingof said switc member, and the flux traversing said magnetism having a.

and an accelerating. switch for governing.

stationary switch contact members, movable magnetizable members,

when the motor current:

said flux path of said series coil being adapted to maintain said switcmember open, whereby variations due to voltage or temperature of saidshunt coil aflecting the fluxes traversin both said normal flux path andthe coinciding part of the flux path of said series coil do not affectthe control of said member.

24. In a switch, the combination with a core member and a movable switchmember, of a lookout coil and a closing coil, fluxes of the latterexerting opposing forces upon the movable switch member, and means sopositioncd as to effect the closing of said switch at a predeterminedcurrent flow in said lockout coil, irrespective of temperature andvoltage changes in said closing coil.

An electromagnetic switch having a movable switch member,electromagnetic means for closing said member and for compensating forvariations in the degree of energization of said electromagnetic means,and electromagnetic means for opposing the movement of said member.

26. An electromagnetic switch having a movable member, electromagneticmeans comprising a shunt coil for actuating the same and forcompensating for variations in the actuating force because of variationsin the value of the current traversing said coil, and a series lockoutelectromagnet for opposing t'he movement of said member.

27. An electromagnetic switch having a movable switch member, anelectromagnet comprising a series coil tending to retain said member inits open position, and electromagnetic means for closing said member andfor compensating for variations in the energization of saidelectromagnetic means whereby said switch member is actuated to itsclosed position When the current traversing the series coil falls to apredetermined value, irrespective of temperature and volt. age changesin said electromagnetic means.

28. An electromagnetic switch having a movable switch member, anelectromagnet comprising a series coil tending to retain said member inits open position, and electromagnetic means comprising a shunt coil forclosing said member and for compensating for variations in the ener'zation of said electromagnetic means y said switch member is actuatedto its closed position when the current traversin the series coil fallsto a predetermined va ue, irrespective of temperature and voltagechanges in said electromagnetic means. 7

In testimony whereof, I have hereunto subscribed my name this 17th dayof May HARRY A. LEWIS. Witnesses: V

G. CARROLL HoovER, R. W. MCGARVEY.

